(1) Field of the Invention
The present invention relates to the sterilization of containers and particularly to a wet aseptic technique for rendering plastic containers virtually free of live micro-organisms whereby such containers are suitable for use in the storage of food stuffs. More specifically, this invention is directed to sterilizing apparatus and especially to apparatus for reliably and substantially completely coating the interior surfaces of open-topped pots and the like with a material which functions as a germicidal agent. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well-suited for use in the sterilization of open-topped plastic containers of the type which are commonly employed in the retail distribution of dairy products. In order to render the use thereof practical, such containers must be treated so as to be substantially free of live micro-organisms at the time of filling. Micro-organisms, if present, will expedite the spoilage of the product with which the container is filled and thus lead to unacceptably short shelf life. There are a number of techniques available in the prior art for killing micro-organisms in containers destined to receive food-stuffs. These prior art techniques include irradiation with ultraviolet light, use of a pressurized air-steam mixture, use of a sterilizing gas, and the so-called "wet aseptic" processes. In prior art wet aseptic processes the interior wall surfaces of a container to be sterilized are sprayed with an atomized liquid which is unstable in vivo and transient in action in vivo. The sterilizing agents used in prior wet aseptic processes include solutions of hydrogen peroxide, alcohol and chlorine. The spraying step, in prior wet aseptic processes, is followed by drying, typically with a stream of hot air. Wet aseptic processes are disclosed in U.S. Pat. Nos. 3,723,060, 4,099,914, 4,169,123 4,296,068 and 4,424,189.
Of the available prior art sterilizing techniques, the wet aseptic process wherein a peroxide is employed as the sterilizing agent has proved to be highly reliable and containers sterilized by this process have exhibited the maximum period in which food-stuffs may be stored without spoiling. The highly effective germicidal action of peroxide is attributable to the fact that atomic oxygen is formed during decomposition of the peroxide. Atomic oxygen exhibits a very strong oxidizing action at the moment of formation and thus is an exceptionally effective bactericide. The organic substances, for example the micro-organisms themselves, actually catalyse and thus take part in the decomposition reaction which results in the formation of the atomic oxygen. The effectiveness of a wet aseptic process employing H.sub.2 O.sub.2 is further enhanced by the fact that liquid hydrogen peroxide, which condenses on the container surfaces, penetrates into the cells of micro-organisms and causes the destruction thereof.
When a germicidal agent is distributed by spraying, only relatively slight wetting of the interior surface of an open-topped container can be achieved. The inability to obtain complete wetting is attributable to the fact that conventional spray nozzles produce droplets which have diameters in the range of between 50 and 150 .mu.m. Accordingly, when the droplets form on the surface of a container during a spraying operation, relatively large areas will remain between the droplets and these areas will be unwetted and thus not sterilized. This problem, i.e., the inability to completely wet the surface or surfaces of a container to be sterilized, is aggravated by the facts that it is generally desired to spray only very small amounts of a H.sub.2 O.sub.2 or similar solution, i.e., 10 to 30 mg per average container, and the containers are typically comprised of a plastic material which is either unwettable or can be wetted only with difficulty.
Subsequent to the spraying step a wet aseptic process wherein peroxide is employed as the germicidal agent, it is common practice to perform a further treatment step to insure that only extremely small amounts of peroxide, which is a toxic substance, will remain on the surfaces of the container. The required removal of the peroxide will be accomplished by the volatilization thereof through the application of heat. Accordingly, when using a wet aseptic process and employing a hydrogen peroxide solution, an after-treatment is invariably required in order to remove unacceptable amounts of peroxide which remain after the sterilization step. Thus, in addition to the inability to obtain absolute sterilization, because of the formation of relatively large droplets as discussed above, the prior art wet aseptic techniques have also been characterized by a comparatively high consumption of energy and by a high consumption of the sterilizing agent itself.
In summary, there has been a long-standing desire in the art to improve the efficiency of wet aseptic sterilizing techniques and particularly to insure that substantially the entire surface of a container to be sterilized is wetted by the sterilizing agent while simultaneously reducing the consumption of the sterilizing agent itself and reducing the energy requirements of the sterilizing process as a whole. Optimally, a wet aseptic process which makes use of a sterilizing agent having less toxicity when compared to peroxide, while achieving the above-discussed improvements in efficiency, has long been sought as a desirable substitute for previous processes.